Hi, Luboš awesome blog. I wish I understood half of this particular post. I like that you despite defining yourself as a string theorist, you keep an open mind and seem ready to accept other concepts as long as they seem to make sense.

Here is the picture I got from Erik's recent talks.

We can describe the universe by a matrix with D-branes on the main diagonal and open-strings off the diagonal. We can put the state of the universe into approximately block-diagonal form. Black holes are blocks in which diagonal and off-diagonal entries are in equilibrium. When matter outside a black hole falls in, the matrix elements connecting the matter block to the black hole also enter equilibrium, and the black hole grows. Finally, for the gravitational interaction between two objects outside a black hole, we integrate out the matrix elements connecting their blocks (the open strings).

I may be fuzzy on the details, but isn't all of that perfectly orthodox?

I then understand Erik to be saying two things. First, the growth of a black hole is an entropic process, occurring in a phase space of volume exp(A/4G). Second, the ordinary gravitational interaction is not an entropic force, but the matter degrees of freedom on the diagonal do still "feel" the volume of the off-diagonal phase space, through something like a Berry phase, and this determines the effective form of the interaction. (A/4G)v is Erik's guess as to the log of this volume. (v must be dimensionless, i.e. it's v/c.) A horizon forms when v=c, and the diagonal degrees of freedom go into equilibrium with the off-diagonal degrees of freedom.

Dear Lumo,
I think that you are taking too many things that we think are correct as facts. Your arrogance is overwhelming. Einstein would have trouble with your thoughts.
I think that Eric's theory needs scrutiny and more work. I do not agree that it is completely wrong.

Cheers,
Joel

Dear Agno,

sorry, no, but I don't agree with a single well formulated statement of yours.

It's not true that one particle in the Universe inevitably creates some entropy. The entropy is the logarithm of the macroscopically indistinguishable microstates, and if there's only one such state, the entropy is just zero.

It's equally untrue that there exist processes that "reduce" the entropy. The total entropy never decreases, at least not by more than ΔS with probability exceeding exp(-ΔS/k) which is de facto zero (probability) if ΔS is macroscopic - note that k is Boltzmann's constant which is 1.38 x 10^{-23} J/K.

The entropic force only occurs if there are many particles (or degrees of freedom, to be general) with many possible states and a difference in entropy, and the term "entropic force" is just an alternative, approximate, vague, overall way to describe the situation. The microscopic origin of an entropic force always has to depend on some fundamental forces - e.g. gravity, electromagnetism, or elastic collisions.

I can't make sense of the rest of your text but because you're talking about the reduction of black hole entropy and similar gems, I am afraid that I am no losing anything by not understanding what you're saying.

Best wishes
Lubos

The particle doesn't really split. The wave function that is useful to describe what will happen has two components because it is nonzero near both slits.

But the wave function is not a real wave, and it is not a divided or diluted particle, either. It is a tool summarizing the information whose only physical meaning is that the squared absolute value of this wave function determines the probability density that the particle will be observed around a given point. So various terms in this wave function interfere with each other, but the wave itself is not a material object, and the particle will be detected at a particular place whose location is only known probabilistically.

The "reduction" of the wave right after the particle is detected is not a real physical process because the wave is not. It's just a simplifying psychological exercise we can do once we know that some probabilities have turned into 100% facts, and the probabilities of competing statements dropped to 0%. So there's no problem with the wave's shrinking "immediately", faster than light, because only objects that can influence others can't propagate faster than light. But the "wrong" portion of the wave function couldn't influence anything, as we know once the particle is observed elsewhere. ;)

This is the full and real story. Whoever gives "either ... or ..." options so that the right answer doesn't appear among the options is simply wrong. Interfering particles behave neither like classical particles nor like classical waves or classical fog or classical anything. They behave according to their own quantum ways.

IMHO, Gravity is entropic only around black holes ( even around micro Ball Lightning) because Fermions are not only repelled from the BH horizon but even created there ( pair production) ( according to the Quantum FFF Hypothesis)
No fermion ( mass) created gravitons can escape from the black hole nucleus.
Only the pushing oscillating Axion/Higgs field vacuum is sucked in to the growing BH nucleus.
see: http://bigbang-entanglement...

The same problems with entropic gravity occur for stars as well as black holes. By locality, it's obviously irrelevant for a distant satellite whether the source of the gravitational field has an event horizon or not.

Regarding the Slit experiment.

To be honest there is no issue.
As the interference happens where both particles meet up in the same point in space and therefore have the exact same entropy.

Be sure that there is a lethal problem. The evolution from the initial state to the detection of the interference pattern must proceed through trajectories that visit points of the configuration space with different heights of the neutron. When these points of the configuration space are assigned different entropies, there can't be any one-to-one map between the microstates at different neutron's heights, and when there's no one-to-one map, it proves that the system cannot follow the one-particle Schrödinger equation because the latter *does* have a simple one-to-one map between the states with different heights of the neutron.

For a more complex argument saying the same about the impossibility to get the right equation for the wave function, see e.g.

http://journals.aps.org/prd...
https://arxiv.org/abs/1009....
https://arxiv.org/abs/1108....

It's obviously not true that the same entropy at the same point - a tautology - is enough to reproduce the interference. The interference requires an evolution matching the equations for propagation of waves, those equations work in a Hilbert space in which X,P of the neutron are well-defined operators, and it implies that translations by delta X - operator exp(i * delta X * Phat) - must be one-to-one maps acting on a Hilbert space.

https://www.quora.com/How-v...

How do you know all the information that you have is the truth?

May I ask you some fundamental values that you hold?

Do you believe in GOD?

God? I had no need of that hypothesis in the blog post above.

You see, Mitchell, that my guess was right on the money. Every detail of your behavior betrayed you. ;-)

Yes Lubos, I am imprisoned in a Dutch crack house and I don't get my fix unless I promote Erik's theory. But I'm not doing a good enough job, so they're going to give me a few days to get my act together.

I have no idea what to do with these sentences. This is just a sequence of pure rubbish. A black hole maximizes the entropy among the bound states of the same mass and charges and angular momenta, so when you create it, you maximize the entropy. When you don't create it, you don't quite maximize it. What else do you think you can say about these obvious things that hold for every system - and its highest-entropy macrostate - and that have been known for 100+ years?

Moreover, there are no "fast degrees of freedom" describing two frozen planets orbiting each other. There are no fast degrees of freedom at all because the system's entropy vanishes. So these non-existent fast degrees of freedom can't be responsible for anything. But as I explained, even if there were fast degrees of freedom, the total amount of phase space corresponding to these fast degrees of freedom would have to be totally independent of the values of the slow degrees of freedom, by unitarity and reversibility.

I can't get rid of the feeling that you have decided to become a mindless parrot of some complete and self-evident crackpottery. Have you been paid for that? Or do you really fail to see that what you keep no writing is just pure rubbish?

Your last short comment makes no sense, either. First, you seem to have restored the speed of light as a quantity not equal to one. But why didn't you do the same thing with hbar and k_Boltzmann? Moreover, your power of c is incorrect. The actual entropy in normal units is

S = A.c^3.k_{Boltzmann}/(4.hbar.G)

So your powers of k, hbar, as well as c are just wrong. But even if all those exponents were not wrong, what the hell are you saying? Another detail is that the phase space corresponding to a black hole has volume not A/4G but rather exp(A/4G), in the c=G=hbar=k=1 units.

Sorry, you must be completely drunk or on crack right now because I know that your texts are usually more coherent than this one.

It's surely nonsense that the entropy gradually grows as your "proportionality to escape velocity v" suggests. There is nothing continuous about the evolution of the entropy. The current entropy of the Sun is 2 x 10^{63}. If it were to collapse to a black hole, its radius would be a few kilometers - 10^{39} Planck lengths or so - so the entropy after the collapse would abruptly jump to 10^{78} or so, by 15 orders of magnitude. The entropy of a star is parameterically negligible relatively to a black hole entropy: after all, it typically scales like A^{3/4} only instead of A. You should first learn these basics of physics before you decide to promote your opinion about a would-be ambitious (crackpot) statements about quantum gravity.

Cheers
LM

Dear Mitchell,

you write: "I still think there must be an insight there which is both not trivial and not wrong. :-)"

Is that your primary assumption? That would explain why nothing you wrote makes any sense to me because this assumption of yours is probably completely wrong.

Some configurations of matter - such as the Sun - don't collapse into a black hole. Others do. When they do, the huge black hole entropy is produced during the time when the horizon gets formed. What's the problem?

Why would you describe the simple process of a formation of a black hole by - seemingly deliberately - cryptic sentences such as "phase space containing the black hole is a region of convergence for many dynamical trajectories"?

The phase space of a black hole corresponds to microstates of a black hole which are the generic microstates of a localized matter with the same value of the mass and charges. So once you get to the path when the black hole is formed, you're inevitably visiting the whole phase space. However, for (low-density) objects such as the Sun, there is a barrier that prevents you from the formation of a black hole. Instead, you are kept in a tiny subspace of the phase space, macroscopically known as the "state of the Sun", so there's no gravitational entropy, so there's no justification to talk about it.

You: "Erik isn't saying any more that gravity outside of a horizon is maximizing an entropy, but he's saying that its form is determined by conservation of the phase space volume for the fast degrees of freedom, and when slow and fast timescales converge, you get horizons and a transition from a kinetic to a thermodynamic regime."

As a result of this discussion, I think I have at least identified the crucial proposition of "entropic gravity 2.0", which is that the phase space volume for the fast variables, associated with a uniform gravitational potential surface on which the escape velocity is "v", should be (A/4G)v. When v=c, a horizon forms, and the thermodynamic regime takes over.

Hopefully we can agree that the growth of a horizon is an entropy-maximizing process. So what about horizon formation?

Consider a black hole of a given size. It has a large gravitational entropy. Now consider various ways that it could have formed (gravitational collapse of a gas cloud, infall of gas onto a neutron star, collision of two neutron stars, etc). In each of these situations, before a horizon forms, there is negligible gravitational entropy. The region of phase space containing the black hole is a region of convergence for many dynamical trajectories, which is how a system moving along any such trajectory can experience a big jump in total entropy when the horizon forms. It's entered a bigger region of phase space.

Somehow there is a transition from a situation (no black hole) in which maximization of gravitational entropy plays no dynamical role, to a situation in which it does, because the black hole now exists and its horizon can grow. Erik isn't saying any more that gravity outside of a horizon is maximizing an entropy, but he's saying that its form is determined by conservation of the phase space volume for the fast degrees of freedom, and when slow and fast timescales converge, you get horizons and a transition from a kinetic to a thermodynamic regime.

I still think there must be an insight there which is both not trivial and not wrong. :-)

Dear Mitchell, you're fooling yourself all the time.

In a unitary quantum theory - and even in a classical theory with a time-dependent "overall" Hamiltonian for all degrees of freedom - the volume of the phase space below some energy (or, quantum mechanically, the number of microstates in it) is *exactly* conserved.

This volume actually doesn't depend on dynamics at all: it is a universal function of the overall energy and the overall energy is exactly conserved. Because this phase space has a constant volume, the "entropy" calculated from this volume in Verlinde's sloppy way (as a logarithm) is also constant which means that the force is zero.

There's really no way - except for introducing errors into physics by hand - how you could get an entropy-based force that would still lead to reversible phenonomena. It's a contradiction. Any dynamics based on entropy differences is irreversible by the first key property of the entropy, the second law of thermodynamics.

Regardless of tiny mutations of the wording, you still haven't gotten rid of the basic error that was present in this Verlinde stuff from the beginning. You still seem to think that the number of microstates - or volume of phase space - corresponding to the Sun and the Earth at different distances depends on the distance (maybe even exponentially with a huge extra coefficient in the exponent).

But this is impossible: it would violate unitarity in a dramatic way. The Sun and the Earth are getting closer and further every year, so whatever microstate for the fast degrees of freedom you have in January when the Sun-Earth distance is minimal must also have their counterparts, in a one-to-one fashion, in July when the distance is maximal. So the phase space volume beneath "E" is obviously independent on how you realize "E" - pretty much by definition. In particular, it must be independent of the Sun-Earth distance, otherwise this distance couldn't change in both ways in a unitary fashion.

Much more strongly, we know from physics - obviously from string theory but we don't really need string theory - that the Sun-Earth system carries no gravitational entropy, surely no entropy that would scale like A/4G with a macroscopic value of A - i.e. an entropy comparable to the black holes of macroscopic dimensions. The Sun and the Earth only carry a very tiny entropy, essentially equal to the number of atoms in them, and this entropy has clearly nothing to do with gravity. In principle, you could consider frozen planets orbiting each other. The gravity wouldn't change but the entropy would be (nearly) zero.

Only event horizons may produce a large entropy of order A/4G. No horizons means no entropy. I can't believe that this elementary point known from the early 1970s is still being misunderstood in 2011.

Cheers
LM

Hi Lubos - I understood Erik to be saying that different degrees of freedom could spontaneously develop different characteristic timescales of evolution, and that when you construct an effective theory for the slow degrees of freedom, it will contain these reaction forces. The phase-space volume beneath a given energy level is very nearly conserved, and the log of this volume is his "entropy".

Dear Mitchell, sorry to say but what you write (and what Verlinde is saying these days) still makes no sense whatsoever. It's just masking the crackpottery he has been previously spreading under a thick layer of fog.

The 2007 paper you referred to is about Berry's phase which is a quantum phase induced by an adiabatic change of parameters. But the adiabatic change of parameters must be pushed by another, external force: the adiabatic change is not the first driver. The external parameters are being slowly changed by an external agent so that the entropy doesn't increase - that's what we mean by "adiabatically" - and we may study the reactions of a particular system to these slow external changes. But the slow external changes are driven by an independent force, e.g. by muscles of a human controlled by the human's "free will": there is no "formula" for "what the adiabatic changes should be". The "spontaneous" force acting on the parameters - without an independent driving force - is zero.

The same is true for any other paper about the "adiabatic reaction force" in this context, e.g. one by Berry himself (with Shukla).

The term "adiabatic reaction force" was just used by Verlinde to replace "entropic force" because everyone has understood by now that gravity isn't an entropic force, also because entropic forces are *irreversible*. But when the entropy actually isn't increasing, the entropic force goes to zero.

So saying that the "adiabatic reaction force" is nonzero is the same childish mistake as talking about "forces induced by a constant gravitational or electric potential". By a simple calculation, d0/dx = 0, we get zero. You can't have it both ways. Forces related to entropy are either irreversible, or zero. Each of these conditions is incompatible with basic properties of gravity.

Cheers
LM

Another important detail is that it's only a formal "entropy" and "temperature" appearing in Erik's equations. There's something non-standard about the meaning of adiabaticity in the context of reaction forces, which I'm trying to pin down. But here's a quote:

"In thermodynamics 'adiabatic' is used in a macroscopic sense to refer to a process occurring
in a thermally insulated system, so that there is no flow of heat to or from the surroundings.

"In reaction dynamics, the word has been used in a microscopic sense, with a range of meanings which have only a tenuous relationship to the thermodynamic meaning or the etymology. Whereas the thermodynamic meaning relates to conditions imposed on a process by an observer, the microscopic meaning relates to conditions under which the process occurs
naturally."

Lubos, now that entropic gravity is no longer entropic, it makes sense! :-)

The claim now is that gravity away from horizons is an adiabatic reaction force, which degenerates into a true entropic force at horizons, where the adiabatic approximation breaks down. When Erik said that the entropic force doesn't increase entropy, he meant that entropy is an adiabatic invariant for the reaction force.

arxiv:0709.2136 derives gravitational precession within an adiabatic approximation and made the whole thing much more plausible to me.

As for Erik's new cosmology, he admitted in discussions at Perimeter Institute that he was uncertain about a lot of details. I think there's still a great possibility that some version of these ideas will provide a unified holographic explanation for dark matter and dark energy.

Dear Joel, do you actually think that you have found any imperfection or possible loophole in my multiple proofs that Verlinde's staements can't be right, or did you just come here to spread bitterness and fog without having a clue?

Do you actually think that your content-free emotional scream may be viewed as an answer to my thoughtful analysis?

The statements have already undergone scrutiny and no other serious physicists think they're correct which is why papers are only being written by authors who are not exactly 1st or 2nd class physicists.

More work is the last thing that this hyped nonsense needs. The first it needs is less hype i.e. less work because there is nothing to work on here. It's just a collection of a few statements that may be easily shown to be incorrect if one looks what is actually being said.

Cheers
LM

Dear nemo,

a completely reversible entropic force is impossible in the real world by the very definition of the entropic force: it is the force driving the system to the loci of the configuration space where the entropy is higher.

One can only consider an idealized concept of an isoentropic process - or reversible adiabatic process. But only processes that occur "infinitely slowly" can be approximated in this way. A general interaction between objects moving and changing by finite speeds - or even speeds comparable to the speed of light - cannot be described as an adiabatic process by any stretch of imagination.

These are absolutely elementary facts about thermodynamics that undergraduate students learn in their thermodynamic courses. Also, you find these facts written in all fair introductions to thermodynamics. A random page on Wikipedia, Joule expansion, for example correctly states that a route that keeps entropy constant (i.e. keeps thermal equilibrium at all times) "can only be realized in the limit where the changes happen infinitely slowly."

The fact that Erik Verlinde or Mr Hammock are ignorant about these elementary things don't make them any less elementary.

Isoentropic (or constant entropy) processes require all the bodies that are in contact to have the same temperature. That's explicitly violated in Verlinde's picture where the temperature is determined on pairwise relationships between the objects. There would surely be lots of heat transfer and entropy increase which is an irreversible process.

The entropy changes that Erik Verlinde would need to make the gravity work would not only be nonzero: they would be astronomical, comparable to the black hole entropy which is the largest entropy that a fixed-mass localized physical system can have. LeSage's defunct theory of gravity died for the same reason: it predicted friction that would slow objects down: that's a manifestation of irreversibility. The advantage of LeSage relatively to Verlinde was that the coefficient in front of the entropy was adjustable in LeSage's picture so one could hope that the produced entropy could be made small (which didn't really work, either, but there was a hope): however, the entropy in Erik's picture is of the order of the black hole entropy for the same masses, so it is surely not small in any case. The induced irreversibility would be gigantic and it would abruptly stop all motion.

Hasn't this already been discussed many times on this thread?

Best wishes
Lubos

in this post

http://www.science20.com/ha...

i read this:

"One such aspect has to do with the question of the reversibility of entropic gravity forces. Some have claimed that entropic forces are necessarily irreversible. In other words, in a system with entropic gravity it would in general not be possible to cause the system to trace back its history.

Reversible Entropic Forces

The claim that entropic forces are necessarily irreversible is a misconception. Verlinde has tried to argue against this wrong concept, but he failed to note a key point. Key is that if the underlying microscopic (Planck scale) dynamics is reversibility, so will be the emerging entropic force."

your comment ?

Dear Mitchell, thanks.

Yes, of course, I noticed the article. It's pretty bad. Dennis Overbye who was at the top of my U.S. MSM science journalists list dropped by 1 place immediately.

Raphael Bousso politely tells him that after a journal club, everyone at Berkeley - including the early enthusiasts - would know that Erik's picture made no sense.

Dennis didn't know what this comment meant. Instead, he was building on hyperpolite clichés by Andy Strominger and others who say that they love similar crackpot papers because they support discussions etc.

Well, if discussions are the main or only real value of that job, why doesn't Andy discuss papers by Lee Smolin, Jack Sarfatti, or any other crackpot, for that matter, all the time?

It's bad that the selection of good ideas and filtering of nonsense is defunct - and maybe even reverted - when it comes to modern sources of influence such as the newspapers. Arbitrarily bad rubbish gets promoted if its author together with some journalists have the interest to do so and are given the freedom to cherry-pick and distort the testimonies of others - which they almost always do.

Erik Verlinde can't possibly believe the dumb things he is saying today - that gravity doesn't exist for him, and so on. I guess that if you bring him to the window and tell him "jump", his acts will still show that he does believe in the existence of gravity, after all.

Best wishes
Lubos

Dennis Overbye just published an article in the New York Times about this theory. Unfortunately they have already closed the comment section, so I couldn't post a link to this page. There are over 200 comments, but apparently none of them from people who read the paper and understood it.

Dear nymodernism,

you're not "sure" whether Verlinde's and LeSage's theories are "similar" because you don't have the slightest clue about physics. You're just an obnoxious troll.

I have explained that these two theories are not just "similar": LeSage's theory is a special case of Verlinde's theory. And it is very easy to see why. Here is the reason, once again.

In LeSage's theory, objects A and B are attracted because in between them, there is a "shadow" of LeSage's particles. The relative excess of LeSage's particles on the external side pushes objects A,B towards one another.

This whole thing therefore relies on the non-uniformity of
the distribution of the LeSage's particles that is larger when the gravitational potential energy is larger (objects are further from each other). When this non-uniformity goes away, the force of gravity disappears, too.

This non-uniformity means that the entropy is not yet maximized. The entropy tries to maximize itself by pushing the objects towards each other. When all objects coincide, the distribution of the LeSage particles in space becomes uniform and their entropy is maximized.

So these are the very same kind of theories and one can formulate them in such a way that it's not just a similarity: they're in the same universality class.

These theories are excluded because they predict many unobserved effects such as irreversibility - which has been known to be a deadly problem of LeSage's theory for more than 200 years but some people have forgotten about this basic physics.

Another problem is that the real gravity, as we observe it e.g. in neutron interfereometry, acts in agreement with the equivalence principle and with the laws of Newton or Einstein even on individual particles. However, LeSage/Verlinde's force could only converge to a particular macroscopic formula after the force is averaged over an ensemble of particles. That's much weaker an agreement with gravity than what has been supported experimentally.

Yes, I was the first one who wrote this criticism in the context of Verlinde's theory. But that's no reason to be ashamed. ;-) I am right and every person with the basic knowledge of physics must know and does know that I am right and Verlinde is wrong.

Best wishes
Lubos

PS. I am not even sure LeSage's and Verlinde's theories are similar in some ways. This seem to be something you have made up yourself, only to associate Verlinde with an idea that is still popular in pseudoscience. Your behavior is not serious.

Lubo: I am sory to say this, but you really don't understand what you talking about:

"It is really the same story [LeSage's and Verlinde's theories]. While the LeSage theory is considered a defunct 17th century pseudoscience, its generalization by Erik is a nwe hot kid on the block for Nude Socialists and others. ;-)"

LeSage founded his theory (wich was no pseudoscience, only a hypothese that failed) on the corpuscular wiew on matter and particles; ie, it consisted on hard material particles. Verlinde talks about particles in a modern sense. Predictions from the two theories can't be the same, and it can't be "the same story".

It is also a bit ironic that you behave like you are a much better and smarter scientist then Verlinde; wich you certainly not are. It is a good idea to cool down and try to reduce the air in your ego a little.

"But I don't understand how Erik Verlinde, an expert, could overlook all the things you describe, if the situation is so clear."

Neither do I.

"AdS/CFT involves supergravity in the bulk decoupling from what happens on the branes."

Apologies but you have mixed these things up. AdS/CFT says that dynamics on the brane is *equivalent* to the gravitational physics in the AdS bulk. They not only fail to be "decoupled" but they're identical. All their degrees of freedom overlap: they're just being translated from one side to the other side.

What decouples in the context of the AdS/CFT, when you're deriving the AdS near-horizon limit of the D3-brane geometry, is the gravitational physics inside the bulk asymptotically Minkowski space, where the D3-branes were originally inserted, from the physics in the (conformally/causally) infinitely long throats near the D3-brane horizons. The physics inside these throats turns out to be equivalent to the "gauge" physics described by the D3-brane Yang-Mills-like dynamics.

Then you're mixing other things, too. When two objects attract, there surely *is* some interaction. But that doesn't mean that there's decoherence. Decoherence only occurs when there are infinitely many "environmental degrees of freedom" coupled to the system you study. But as I explained above, simple pairs of two massive objects at separation R don't have a large number of states. There are no environmental degrees of freedom over there, so there's no decoherence. But this fact has nothing to do with decoupling or the lack of interactions. Indeed, everything inside the CFT - or everything inside the bulk AdS space - interacts with everything else as much as it can.

It seems that you figured that out, too.

I understand the interference argument. But I don't understand how Erik Verlinde, an expert, could overlook all the things you describe, if the situation is so clear.

I have been reading Maldacena and Witten's papers from 1998 and a few others, identifying the gaps in my knowledge. Maybe I should just shut up until I actually understand the correspondence. I don't want to be one of the ignorant loudmouths stating arbitrary opinions based on misunderstandings. However, I will record one more set of thoughts I had.

Immediately after I posted the earlier comment, I thought: "AdS/CFT involves supergravity in the bulk decoupling from what happens on the branes. If there's no coupling, there's no interaction, no decoherence of gauge interactions by gravitational degrees of freedom, and neutron interferometry can still work! Could that be part of Verlinde's idea?"

Then I had a second thought: "But the neutron in the experiment still feels the gravitational field. So doesn't that imply some interaction?" And at that point I know that further private speculation is fruitless until I actually understand how geometries can be built out of closed string states (and until I know many other things).

Dear Mitchell, Erik's assumptions are simply not true in AdS/CFT - or any realization of quantum gravity. If you create two large objects in the bulk AdS space, the number of microstates will not depend on the separation, certainly not in the violent way Erik assumes. Among objects in volume with area A, only black holes have entropy of order A/4G. Erik assumes that any pair of objects will have a similar entropy.

One can explicitly show that just like the assumptions are incompatible with AdS/CFT, so are the deduced consequences. Interference between gravitating objects doesn't get lost, the gravitational force is not irreversible, and so on. This is trivial to see. One can create microstates in the CFT corresponding e.g. two BPS but mutually non-BPS objects that gravitationally attract. They will be unique, independent of the separation, and one can also explicitly see that the interference won't get lost. Interference patterns never get lost if we have just a couple of states that are labeled at most by one relative position because the mathematics of the QM calculation remains isomorphic.

I was assuming that such things are self-evident to all the readers. Thinking that interference gets lost for no good reason is just a basic error in thinking. I can't add the comment "2+2 doesn't cease to 4" and millions of similar comments to every sentence. Certain things are simply always true.

Is it possible to identify *where* Erik goes wrong in his thinking about AdS/CFT? This argument shows he made a mistake but it doesn't tell us where.

I thought of making an AdS analogue for the double-slit experiment - maybe gravitational lensing of a test particle by AdS black holes - and then examining the CFT description. But that is way beyond what I can do now.

Lubos,

Some clarification for Neutron experiment you were referring.

Best,

First, thanks for your time and reply.

Second, I am very sorry, but I certainly wasn't referring to you as individual, but to physicists attitude in general perceived to a layman, myself included. It is my general notion that majority of great minds throughout the history of physics were always too quick to dismiss other people's theories, and almost infinitely stubborn in their own. Surely, I didn't mean any disrespect to you personally in any way and, again, I am sorry if it was interpreted as such.

Now, back to LeSage. Thanks for clearing CMB radiation, but how can we really exclude the possibility of ever finding out any such particle? I find it easier to believe in non-friction inducing mass absorbed currently undetectable particle than many other generally accepted but only theorized phenomena.

Didn't we see frictionless fluid motions at extremely low temperatures?

Also, what about impact of all macroscopic objects (comets, space dirt, rocks) or even free floating microscopic ones (molecules of gas)? Shouldn't that slow down Earth then?

Furthermore, shouldn't friction actually increase orbiting speed, but only reduce orbital potential energy?

Yes, theory was killed 300 years ago, but I consider that to be in favor of re-mentioning it.

Please feel free to disregard this reply if you think it is inappropriate for you common audience or you just don't have time to explain physic basics to random visitor. I'm pretty sure I am way over my head here. :)

Thanks again and kind regards,

Looka

Dear Looka,

you ask: "Well, isn't this true even without Le Sages particles? Space is not that empty, is it?"

It's definitely not true without Le Sage's particles. The vacuum in quantum field theory (or string theory) is "not empty" but you must understand what the "non-emptiness" actually is. It is filled with virtual particles, not real particles.

Virtual particles only last for a finite time, so if they're able to change the momentum of an object, the change is abruptly "undone" as soon as the virtual particle ceases to exist. One of the consequences is that there is no "slowing down" of objects in the vacuum. A stable object with a velocity "v" will have the same velocity forever: the momentum is conserved. This is exactly true in quantum field theory, even when all of its subtle phenomena are introduced, and we also know experimentally that it has to be true. Any hypothesis that violates this principle of inertia is instantly falsified.

Your question: "Isn't it a fact that every little part of universe is actually filled with all kinds of waves/particles going in every possible direction?"

Again, yes, but they're virtual particles and they have none of the effects you incorrectly attribute them because they're not real particles.

You continue: "I mean, where ever you position yourself and whatever direction you look at, even with just naked eye, you will 'see' something. Always. Not even mentioning CMB or neutrinos or such, just photons."

This has nothing to do with the question. CMB is not a part of the vacuum CMB is obviously composed out of real photons, not just virtual photons. But the CMB is completely negligible relatively to gravity. If you used the CMB photons as the Le Sage particles, and you considered the shadows etc., the force that you would get by this would be vastly lower than gravity. Easy to know why - if it were different, we would have to include this extra CMB-LeSage force in every calculation. Moreover, gravity could be turned off just by shielding microwaves - which would be very easy. :-)

You: "I am not a physicist. By any definition. But I always find it funny to see the lengths of imagination you will go through and how easily you look the other way when it suits you."

If you think that there exists just an infinitesimal piece of an asymmetry of my imagination and my looking, it's just because your intelligence is far too low to know what is the right attitude. In other words, your accusations are just about your being an idiot.

You: "I know it's a childish concept, but ratio of shielding effect of object "shadow" being exact 1/r2 is just too appealing to me."

Great but it is a theory that has been excluded for a few hundred years and you can't really revive dead theories in physics unless you modify them. Falsification in physics is eternal, and the LeSage theory is damn dead.

Best wishes
Lubos

"Moving objects will hit a higher number of the particles on the front side than the rear side: the excessive front collisions will therefore slow the object down."

Well, isn't this true even without Le Sages particles? Space is not that empty, is it?

Isn't it a fact that every little part of universe is actually filled with all kinds of waves/particles going in every possible direction? I mean, where ever you position yourself and whatever direction you look at, even with just naked eye, you will 'see' something. Always. Not even mentioning CMB or neutrinos or such, just photons. Is it hard to imagine that earth is shielding "something"? I know it is unusual to consider the Sun as a absorber, but I can imagine that it is a damn good one.

I am not a physicist. By any definition. But I always find it funny to see the lengths of imagination you will go through and how easily you look the other way when it suits you.


I know it's a childish concept, but ratio of shielding effect of object "shadow" being exact 1/r2 is just too appealing to me.

Dear Bookie,

apologies but science is not another religion where just different prophets are being worshiped in the same uncritical irrational way as e.g. in Islam. Science is about impersonal methods to find the objective truth about Nature.

Einstein's opinions about QM were demonstrably wrong - it has actually been demonstrated - and no scientist can be or should be "open-minded" about this question just because Einstein was confused. This has been scientifically settled and whatever Einstein thought or felt about it is only relevant for the historians of science, not for scientists themselves.

If it is necessary for you, I will also write that when it came to quantum mechanics, Einstein was a misguided fool with poor intuition. OK?

On the other hand, I also disagree with your comment that Einstein was crazy. During his miraculous year in particular, he was a smooth bureaucrat in perfect suits and organized haircuts. The idea of Einstein as a crazy chap doesn't refer to the Einstein at the times when he made the most important things. See Clifford's recent posting about this issue. Of course, Einstein was very unusual, even as a young officer, from some other viewpoints, but he didn't fit the category of "average" crazy people in any way. What you write is a historical myth.

I also disagree with your comments about Erik's paper. Erik doesn't suggest that the information has to be stored in binary digits - that would be stupid because the very point of information-based arguments like his argument (and even the correct ones that do exist in physics and that are valid) is that they're independent of the way how the information is stored or interpreted.

That's the whole point of entropy and information as concepts that the "formats" can be converted to each other and none of them is privileged. The idea of information encoded as "bits" all the time is just the idea of people with a very limited imagination who can understand some basics of computer science but who are completely incapable to understand physics because in physics, the information is never stored in these simple, well-defined, isolated bits of information. The Hilbert space for no interesting system in physics is exactly isomorphic to a tensor power of a two-dimensional Hilbert space. Look at Hilbert spaces of a Hydrogen atom, of a molecule, or a vibrating string to get a better idea how the information is stored in the state of physical systems in actual physics, rather than computers - which is an entirely different thing.

Best wishes
Lubos

Hi Lubos, I'll defend the deterministic way of thinking from Einstein's point of view. Einstein himself spent his last years looking for "deterministic rules" that were the underbelly of quantum mechanics, a pill he troubled with swallowing. The idea was that the universe is deterministic on plack scales, but that somehow quantum mechanics would emerge at small scales, and GR would emerge at the largest scales. If Einstein himself thought it was worth devoting his last years to, then any physicist should be open minded to consider such theories less they think Einstein was a misguided fool with poor intuition.

Now to Verlinde. In Verlindes view he does not discuss how the bits operates, it's a complete hand waving maneuver that he does not attempt to solve. Any attack on him for this is probably well deserved. However, he does suggest that these bits though are the smallest possible units that can describe the universe, sorry using atoms and large scale particles doesn't seem to be fundamental enough to me either. They're way too large. Every dollar bill can be divided into 100 pennies, get my drift?

Working with the context of bits and not knowing they're underlying rules, I myself would make a few general assumptions assuming their existence:

1. Bits would have to make up particles. Particles must emerge from this information like a hologram. I tend to think of Magic Eyes at this point where it requires an observer to see the picture encoded in the information.

2. These bits of energy can fluctuate up and down perhaps in some dynamic pattern that follows rules, but hey maybe even following QM rules. Therefore, one might assume they have one degree of freedom.

2. Energy is conserved, so if one bit is fluctuating up, then another must fluctuate down.

Based on this one can apply the equipartition theory to a group of N bits and assign an average energy each bit holds. Assuming that the bits are entropically trying to average their energies out, then his model shows how gravity emerges as these bits average their energies out and approach an average energy.

Although his theory is radical and I myself have considered your points. I would agree with you that he could easily be wrong and you could easily be right. I'm not a good enough physicist to know (hah that's why I own a carpentry business now). However, it is quite remarkable that he gets the correct answers using his model. Some "coincidences" are not so easy to explain away.

So then is "virtual energy" or "holographic energy" real? Perhaps it is something real that exists at plank scales, but perhaps not.

I also tend to take a comprehensive view of the world and consider the philosophical view of the world as well. I often ask myself questions like, "If a tree falls in the forest and nobody is around to hear it does it make a sound? If no living beings exist can the world have an appearance?" These philisophical questions lead me to believe that the universe is nothing but information and it takes an observer to "see it" much like a Magic Eye, so holographic approaches to the universe are very enticing to me in that respect.

Verlinde shouldn't be let off the hook though and I hope that you and others challenge his ideas or prove them right or wrong. We all are seeking the truth and unfortunately to get to that destination means taking every possible path so that we can find the dead ends and turn the other way.

Physics needs bad physicists, just remember that. Bad physicist often get jobs at patent offices b/c they're not good enough. But...they just might be crazy enough...and that's what we need to solve this answer, some crazy thinking.

Dear Zbyněk,

that's right. If you know about some limitations of your future acceleration etc., so that your achieved speed will be usually much smaller than the speed of light, you may also deduce that you're doomed much earlier than the event horizon - and the location of the point of no-return depends on your future abilities. It depends on too many things so that it's not an excessively interesting concept.

Holography may have been deduced from the black hole horizon historically but it's not the same thing.

The pair-creation of two particles near the horizon must occur "simultaneously", because of energy conservation.

Hawking derived the entropy by looking carefully at Bogoliubov transformations - it's a technical transformation with harmonic oscillators in quantum mechanics. Without knowing this math-loaded stuff usual in quantum field theory, I don't think it's possible to understand why black holes have a nonzero temperature - why they radiate as Hawking did. In this way, Hawking only derived the temperature, and deduced the entropy - confirming Bekenstein's prediction - by the laws of thermodynamics.

But Hawking never derived the entropy directly. It's not really possible and the question "where do the degrees of freedom behind the black hole entropy actually reside?" - has been problematic for 25+ years. Strominger and Vafa derived the first proper BH entropy, in a controllable framework (string theory), but because it was a stunningly nontrivial "dual" calculation, their entropy wasn't linked to any horizon in any objective way.

Best wishes
Lubos

hello again
I thought the hologram principle was derived from the black hole horizon
I think my misunderstanding is with the definitions - when I say surely it depends on the velocityI meant as most of us (and majority of particles) do not have the benefit of the speed of light, we will be already doomed much further out and our point of no return will be already outside thepoint you call the actual event horizon.
Concerning the entropy - what I thoughtHawking suggests is:this originates from virtual particle pairs that get separated -one falling in the hole - but theothersurviving particle radiating theHawking radiationwe see -does therefore the temperature not originate from the time of separation - rather then from the time of one partner crossing the horizon, in that sense could there not be a scenario where the first happens before the second and since the second particle is lost forever it would not make any difference to the "outside"
Thanks for your explanations
zbynek

Dear Zbynek, first, I don't understand why you're mixing the black hole horizon with the holographic screens. In general, these are very different objects. The black hole event horizon is inevitably a null hypersurface - its internal geometry is the same as one on the planet that is moving by the speed of light.

Most of your question seems to be concerned with the event horizon. The event horizon is a point of no return. That means that if you get behind it, you will never be able to return in front of it, as long as you will be moving along timelike trajectories (equivalent to v smaller than c in special relativity), which is what every mortal massive object should always be doing. So regardless of the speed, who falls inside the black hole - who has crossed the horizon - is doomed. His life is destined to contain a finite piece in the future only. It's the very point of the event horizon that even if you accelerate at the highest conceivable speed - close to c - you won't be able to escape. You have surely heard it this way. I don't know what you're saying exactly the opposite of this generally well-known fact, and even accompany your patently false proposition by the adverb "surely".

So the existence of the event horizon surely holds for all allowed speeds, and the event horizons surely exist for all black holes, not just the Schwarzschild black holes - the existence of an event horizon is really how a black hole is defined. For rotating black holes, the event horizon will be squeezed, twisted, or toroidal, etc., but it will still exist.

I don't understand how you count the "contribution of particles" to the black hole entropy. As long as a particle is outside the hole, it's not a part of the hole, and its entropy must be counted separately. Once it crosses the horizon, it becomes a part of the black hole, but the black hole has entropy proportional to the area of the event horizon - it's huge - and there exists no meaningful way how to distribute this entropy to the objects living inside the hole (which usually have much lower entropy - for example, a star of the same mass has a parametrically smaller entropy).

Cheers, LM

Nazdar Lubosi,
still tying to comprehend the principle...
the holographic boundry - derived from the black hole horizon issupposed tobe a 2D screen - the horizon is or is said to be the point of no return for a given particle within the grav. field of the hole? - surely this is different for all given velocities and directions of travel and corresponds to the schwarzschild only if travelling at c and centrifugal - for a nonzero speed travelling particle in centripetal dirction it is infinate - since all these particles (eventually) contribute to the black hole entropy the horizon is 3D and as large as the universe - or not?
Zdravim
Zbynek

Dear Synthetic Zero,

Verlinde's *results* are not fully (or partially) quantum mechanical because they're inconsistent with basic facts about quantum mechanics such as the existence of interference patterns in an external field.

But its assumptions surely are fully quantum mechanical. Holography, which he builds upon, only exists in "fully quantum mechanical" theories of gravity.

Whether a particular theory is consistent at all scales is an entirely different question from the question whether the theory is quantum mechanical.

You say a lot of vague things about what we don't know about the most profound questions about spacetime. But Verlinde's idea was exactly meant to remove this fog from gravity, and replace it with something specific. So your fog has nothing whatsoever to do with this discussion.

Also, it's nonsensical to say that "coherence is an artifact" of anything. It is an "artifact" of the most important fact about Nature, namely that it follows the postulates of quantum mechanics. If they are "artifact", then everything is an artifact.

At any rate, once coherence is lost, you can't ever recover it.

To summarize, what you write makes no sense whatsoever, and it would be good if you reduced these contributions somewhat. Thanks a lot.

Lubos

Sorry, I should have been more clear. What I meant is that Verlinde's idea is not fully quantum mechanical in that it depends on what currently passes for quantum gravity theories; i.e., theories that aren't fully quantum mechanical because they only work at certain scales and have other limitations: they aren't complete theories. For example, the holographic principle itself seems to imply some sort of dimensionality to spacetime (even though it "emerges" spacetime, it's assuming things like the boundary of a virtual volume, and so on). To my mind, a fully quantum theory of gravity would ideally not assume boundaries, where spacetime itself would emerge out of a much less structured substrate of some kind. Right now we have a situation where even the problem of quantum observation hasn't been worked out --- what actually happens when anything is observed? What is an interference pattern falling in a gravitational field, in a fundamental sense?

That is to say, existing "quantum" theories of gravity are kludges. In particular I think the notion of coherent superposition may be seen in a more fundamental sense to be an artifact of the mechanism of observation or detection, something that at least in my view hasn't been fully explained (i.e., the preferred basis problem in the Everett interpretation). If there's anything to Verlinde's idea then it might not work to think about superposition using one of the current quantum gravity kludges, rather in a more fundamental theory (whatever that might be) you might be able to preserve the observed effect but via a radically different mechanism. That is to say, I think Verlinde's idea cannot be a complete explanation but perhaps it could be a start to an explanation that could work within a radically different context.

Dear Synthetic zero,

there is nothing such as your "virtual chaos" (things are either chaotic, or not) and consequently, there doesn't exist any loophole of the kind you suggested. You have confused the notions of "statistical physics" and "classical physics". Both of them make interference of quantum particles impossible, but in this particular setup (of Erik Verlinde), the "physics" that destroyed the interference pattern was "statistical physics" i.e. chaos, not "classical physics".

When an effect depends on the identification of macroscopically indistinguishable microstates, i.e. on the concept of entropy, its qualitative behavior is identical in the classical version and quantum version, and it always breaks the coherence.

It's just your illusion - a manifestly incorrect assumption - that I have discussed a "classical version" of Erik Verlinde's picture. Of course that in order to discuss interference from individual particles, I had to discuss - and I did discuss - the "quantum version".

And the quantum version of it does destroy the interference pattern. This is a result of "entropy" - and summation of probabilities over similar states - that occurs in this picture, not a consequence of considering a "classical theory" only.

I have only considered quantum theories in my picture.

The survival of the interference picture is only possible if one first sums the complex probability amplitudes, and then squares the sum's absolute value to obtain probabilities. This procedure is equivalent to the requirement that the force that modifies/shifts the interference pattern has to be fundamental i.e. non-entropic.

You are also wrong concerning your idea that Erik proposed a classical picture. Erik's ideas only make sense in quantum mechanics - look that from his equation (3.5), most equations contain Planck's constant and therefore inherently depend on quantum mechanics. Your idea that this is just some "classical version" that can be "improved" to get a quantum version is a misunderstanding of Erik's picture. Holography itself - his starting point - is a property of quantum gravity only: it doesn't exist in classical gravity.

I clearly have a different thinking than the rest of the world. From my optics, it may take 5 minutes - or at most 1 hour - to see that the precise thing that Erik Verlinde is proposing is wrong. I am flabbergasted by the idea that some people will study it for weeks, months, years, or centuries. People who need more than 1 hour after seeing and thinking about these arguments to see that the idea can't work should quit physics because they have no talent for it.

Cheers
Lubos

Hello Lubos,

I have to admit I haven't thought this through in detail but one thing that occurs to me is that Verlinde's idea is fundamentally classical, so it seems a bit unfair to say that it doesn't work quantum mechanically. I.e., while it turns out that in the interference experiment you cite you get the proper "falling of the interference pattern" and no decoherence, it's not clear to me that a quantum version of Verlinde's idea would necessarily not produce this result. Perhaps, for example, in a quantum version of this the reason you get some sort of "falling of the interference pattern" is because you have to sum up over all possible chaotic configurations, or something of that sort, i.e., that what is happening isn't that chaos is "actually" occurring (which would destroy coherence) but rather some sort of "virtual chaos" happens in many parallel worlds, so to speak. Again, I'm speaking vaguely because no one, not Verlinde or anyone else, has worked out how one would actually use his idea in a truly quantum version of the theory.